1. The Death of Rue: This is often considered the most emotionally devastating moment in the book and the movie. Rue is a young, innocent tribute who becomes Katniss's ally, and her death at the hands of Marvel is a turning point in the story, highlighting the brutality of the Games and the moral complexities of survival.
2. Katniss's and Peeta's "Love Story" is a Manipulation: This reveals the Games' manipulative nature. The romance between Katniss and Peeta is strategically crafted by the Capitol to create drama and interest, and the reality is far more complicated.
3. The Existence of the Rebellion: The presence of a rebellion against the Capitol adds a whole new dimension to the story. This revelation shifts the focus beyond the Games and explores the larger fight for freedom and equality.
4. Katniss's Mockingjay Symbolism: Her transformation into the Mockingjay, a symbol of hope and rebellion, is a significant plot point. It sets her up as a leader figure and underscores the impact she has on the fight for freedom.
5. The Capitol's Brutality and Corruption: Throughout the series, the Capitol's cruelty and corruption are unveiled, showcasing a system built on exploitation and control. This reveals the true nature of Panem and the immense stakes of the rebellion.
Ultimately, the biggest spoiler for you depends on what you consider to be the most impactful and shocking revelation. But, the above spoilers are all pivotal to the story's central themes and impact.